This invention relates to a circuit for detecting a trigger signal for correctly indicating the receiving point of a synthesizing type high frequency receiver.
Prior to the present invention, the trigger signal utilized as the sweep stop signal for an automatic tuning operation of a high frequency receiver, a trigger signal of a muting circuit or as a display signal for displaying a receiving point by means of a luminous diode or an incandescent lamp, has been generated by a circuit as shown in FIG. 1 which comprises a receiver including a high frequency circuit 1 including a pulse generator 1a for generating sweeping clock pulses, a local oscillator 1b comprised of a PLL (phase locked loop) circuit so controlled as to produce stepwise local oscillation signals by the clock pulses and a mixer, not shown, an intermediate frequency amplifier 2, a demodulator 3 for producing an audio frequency signal to the output terminal 4 connected to a utilization circuit such as a loudspeaker, not shown; and a circuit for deriving out the trigger signal including a narrow bandwidth amplifier 5 for amplifying an intermediate frequency signal taken out of the intermediate frequency amplifier 2, a rectifier 6 connected to rectify the output from the narrow bandwidth amplifier 5, a wave shaper 7 which consists, for example, of a Schmidt trigger circuit and an output terminal 8. Alternatively, the trigger signal is formed by an AND gate circuit with one input connected to receive a DC signal derived out from the demodulator and the other input connected to receive a DC signal supplied by said rectifier.
In such a prior art circuit, as the receiving point indicating circuit is caused to misoperate by the output of the rectifier or the voice current demodulated by the demodulator, it has been the usual practice to increase the time constant of the rectifier circuit or to include a time constant circuit having a large time constant in the later stage of the demodulator for the purpose of preventing leakage of the voice current, thus preventing the misoperation of the receiving point indicating circuit. However, in a synthesizing type receiver which utilizes, as its local oscillator, a voltage controlled oscillator whose oscillation frequency is successively varied by a sweeping clock pulse, and the oscillator is tuned by electronic sweeping, as the sweeping speed increases, the receiving frequency is swept transiently in an extremely short time. For this reason, it is difficult to detect DC signal due to the time constant of the circuit and hence to increase the sweep speed.